Smart cards, sometimes referred to as IC cards or RFID cards, are a form of portable data device generally having a plastic substrate with a semiconductor "chip" (i.e., integrated circuit (IC)) disposed therein. The semiconductor chip usually includes memory and processor components for processing digital data such as, for example, program instructions supporting debit/credit transactions, user information, and the like. It is well understood that components of the smart card require connection to a source of operating power, usually DC power, at a level that does not exceed the power capacity of the components and thereby damage the components. Most advantageously, the power is supplied externally from the card so that the card does not require a battery that would increase the thickness of the card. To that end, some smart cards include antenna elements (e.g., one or more inductive coils) for coupling to radiated field(s) produced by devices such as card readers, or data communications terminals. The fields to which the cards are coupled may comprise, for example, radio frequency (RF) power signals modulated with 10% amplitude shift keying (ASK). The smart card can thereby derive power from and detect data from the AC power signals.
The above-described cards are known as "contactless" cards because they can couple to the radiated fields without physically contacting the terminal device, for example by a user simply waving the card within a predetermined range of the terminal device. Also known are "contacted" cards that require a physical connection to a terminal device. Additionally, smart cards exist that are operational in either a contacted mode or a contactless mode. Such cards are equipped with RF receiving circuitry (for contactless operations) and contact elements (for contacted operations) and are commonly referred to as dual mode smart cards.
One problem that exists primarily in the contactless mode of operation stems from the card coupling to fields that vary in intensity at different points in space, causing the card to receive variable amounts of electrical power. For example, a card placed directly adjacent the terminal device can receive an order of magnitude greater power than a card placed at the outer limit of the predetermined range. To compensate for such wide input power variations in contactless cards, it is known to employ a power reception circuit including a shunting device ("shunt regulator") to attempt to regulate the voltage across the antenna coil and prevent damage to the smart card components due to overvoltage.
For example, one known type of power reception circuit includes a shunt regulator placed across the output terminals of a rectifier circuit (e.g., a bridge rectifier) in parallel with a power supply bypass capacitor, the aim of the power reception circuit being to restrict the power supply voltage and current to desired levels. However, a problem with the prior art power reception circuit is that it can produce wide fluctuations in output power (commonly known as power supply ripple), due to capacitor discharge when the rectifying bridge is not conducting. Generally, the amount of power supply ripple is directly proportional to the amount of current drawn through the shunt regulator. Although power supply ripple can be limited by the use of very large bypass capacitors in parallel with the shunt regulator, the use of such large capacitors is undesirable because it can drive up the cost and/or increase the size of the smart card.
Yet another problem associated with contactless smart card operation is that of energy fluctuations caused by the smart card IC. In particular, these energy fluctuations, which can be caused by switching noise or current spikes associated with the IC circuitry, can interfere with the smart card's ability to recover data from the modulated power signal (e.g., modulated with 10% amplitude shift keying (ASK)). If the switching noise is allowed to couple to the ASK modulated power signal, the data signal may become corrupted. Thus, the problem of switching noise must be addressed in order to improve performance during contactless operations.
Accordingly, there is a need for circuitry employable in a device such as a contactless smart card that derives, from an input power signal (e.g., a modulated RF carrier signal), an output power signal suitable for driving processing components of the device. Preferably, the output power signal does not exhibit excessive power supply ripple. There is further a need for a circuitry employable in a device such as a contactless smart card that prevents device circuitry switching noise that otherwise would compromise the ability of the card to receive ASK modulated data from coupling to the modulated power signal. The present invention is directed to satisfying or at least partially satisfying the aforementioned needs.